The company's AAT3140 and AAT3141, which combine a loadswitch with a fractional/doubling (1.5 and 2x) DC/DC converter in a 1 MHz topology that requires few external parts, is one of the first to make provision for the rather wide variations in a typical white LED's forward voltage drop. "Matching LED brightness without wasting power has always been a formidable challenge for cell phone and other portable device manufacturers," says David Brown, senior manager of applications engineering. "When battery voltage drops, drivers typically require that all LEDs in a display run off the less efficient charge pump, even if only one LED requires it. By combining our constant-current source architecture with built-in intelligence, we provide power from the charge pump only to those LEDs which need it." That functionality ultimately removes the vendor's expense in binning white LED devices having matching VF's. This chip provides up to 30 mA on each of four current-source outputs. The outputs may be connected in parallel.

The AAT3140 provides a 32-position logarithmic scale LED brightness control via the company's simple serial control (S2Cwire) digital single-wire system. The AAT3140's constant-current output ensures uniform brightness regardless of differences in LED manufacturing lots. The chip also features a thermal management system to protect the device from short-circuit conditions and integrates soft-start circuitry to prevent excess inrush current during start-up. The AAT3141, with the same features as the AAT3140, also adds addressability. Designers can independently control three LEDs to backlight a main display and one or two LEDs (in parallel) to backlight a sub-display.

AnalogicTech bills its second device, the AAT3112, as the first for white- and RGB-LED flash applications capable of delivering 500 mA of pulsed current. This dual charge-pump, a voltage-doubler, provides a regulated output voltage from a 2.7- to 5.5-volt input range, and each charge-pump can be independently enabled. "One of the primary reasons cell phone manufacturers have not integrated the camera flash function has been the size and cost of the parts," said Erik Ogren, product marketing manager. "Until now designers have had to use DC/DC boost converters and fairly expensive, low profile inductors. The AAT3112 eliminates the need for an inductor, and requires only four small, inexpensive external capacitors." As with the AAT3140/41, this chip, which operates at 750 kHz, includes short-circuit protection and integrated soft-start circuitry.

The third new device, the AAT3190, is also a dual charge-pump. Operating at 1 MHz, it provides a low-profile solution for delivering regulated positive and negative voltages for flat-panel LCDs, OLED, and CCD image sensors. This device, working off a 2.7 to 5-volt input, provides two regulated outputs at up to &plusmn25 volts. As with the other devices, it uses small (external) capacitors, and it does away with inductors.

The AAT3140 and AAT3141, in 12-pin TSOP packages specified for operation over -40 to +85&degC, are priced at $1.95, and $2.09, respectively, in 1k quantities. The AAT3112 comes in a 16-pin 3-by-3 mm QFN package and is priced at $1.41 each. The AAT3190, at 1.73 each, comes in an 8-pin MSOP or 12-pin TSOP.
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